1. Field of the Invention
The present invention relates to a fuel electrode catalyst for fuel cell, an electrode/membrane assembly, and a fuel cell and a fuel cell system provided with the electrode/membrane assembly, which are excellent in carbon monoxide (CO) poisoning resistance.
2. Description of the Related Art
In recent years, fuel cells have attracted attention as a high-efficiency generator. The fuel cells are roughly classified, according to the type of electrolyte used, into a low-temperature operating fuel cell such as alkali type, polymer type or phosphoric acid type and a high-temperature operating fuel cell such as molten carbonate type or solid oxide type. Among them, a polymer fuel cell using an ion-conductive polymer electrolyte membrane as the electrolyte has attracted particular attention as power sources for various purposes including domestic use since a high power density can be obtained in spite of a small size thereof.
FIG. 7 is an exploded sectional view showing a basic structure of a unit cell in such polymer type fuel cell.
An electrode/membrane assembly is formed by bonding an air electrode catalyst layer 32 and a fuel electrode catalyst layer 33 each formed using a platinum carbon-supported catalyst to both principal surfaces of a polymer electrolyte membrane 31, respectively.
An air electrode-side gas diffusion layer 34 and a fuel electrode-side gas diffusion layer 35 each having a structure in which carbon paper is coated with a mixture of carbon black and polytetrafluoroethylene (PTFE) are disposed in opposition to the air electrode catalyst layer 32 and to the fuel electrode catalyst layer 33, respectively. An air electrode 36 and a fuel electrode 37 are thereby constituted.
The gas diffusion layers 34 and 35 have the function of conducting electric current to the outside in addition to the function of passing an oxidizer gas (e.g., air) and a fuel gas, for example, mainly hydrogen, an alcoholic fuel such as methanol or a reformed gas mainly composed of hydrogen which is obtained by reforming a hydrocarbon fuel such as natural gas, city gas, LPG or butane. A unit cell 41 is formed by sandwiching the gas diffusion layer 34 and the gas diffusion layer 35 by one set of separators 40 formed of a conductive and gas-impermeable material, each of the separators including a gas flow passage 38 for distributing reaction gas, formed on the principal surface facing the gas diffusion layer 34 or the gas diffusion layer 35, and a cooling water flow passage 39 for distributing cooling water, formed on the other principal surface.
FIG. 8 is a sectional view showing a basic structure of a polymer fuel cell stack. A number of unit cells 41 are laminated, and the laminated cells are sandwiched between collector plates 42 and further between insulating plates 43 for electric insulation and thermal insulation, clamped by clamp plates 44 for retaining the laminated state with loading, and fastened together by bolts 45 and nuts 47, with the clamp load being applied by disc springs 46.
When a fuel gas containing hydrogen and an oxidizer gas containing oxygen such as air are supplied to the fuel electrode 37 and to the air electrode 36, respectively, a fuel electrode reaction for resolving a hydrogen molecule to hydrogen ions (protons) and electrons takes place in the fuel electrode 37, and an electrochemical reaction for generating water from the oxygen, the hydrogen ions and the electrons takes place in the air electrode 36, whereby electric power is consequently supplied to a load by the electrons moving in an external circuit from the fuel electrode to the air electrode, while water is generated on the air electrode side.Fuel electrode; H2→2H++2e−  (Fuel electrode reaction)Air electrode; 2H++(½)O2+2e−→H2O  (Air electrode reaction)Overall; H2+(½)O2→H2O
The hydrocarbon fuel such as natural gas or city gas contains sulfur, and a reformed gas mainly composed of hydrogen, which is obtained by reforming such fuel, contains carbon monoxide (CO). Therefore, when such reformed gas is directly supplied to the fuel electrode of a cell, platinum catalyst is poisoned thereby. The poisoning of the catalyst by CO inhibits the reaction in the fuel electrode, resulting in deterioration of cell performance.
Therefore, a fuel cell power generation system as a small power source including a desulfurizer, a reformer (RF), a CO converter (SH) for converting carbon monoxide, a CO remover (PROX) for removing carbon monoxide, and a fuel cell for generating electric power through chemical reaction of the resulting hydrogen (reformed gas) with an oxidizer such as atmospheric oxygen is proposed (refer to, for example, Japanese Patent Application Laid-Open Nos. 2003-217620, 2003-217623, and 2000-277137).
FIG. 9 shows a conventional fuel cell power generation system.
A conventional fuel cell power generation system 51 comprises, as shown in FIG. 9, a desulfurizer 54, which desulfurizes hydrocarbon fuel gas such as natural gas, city gas, methanol, LPG or butane supplied thereto through a raw fuel gas supply line 53 provided with a raw fuel gas on-off valve 52; a fuel reforming device 60 [reformer (RF)/CO converter (SH)/CO remover (PROX)], which reforms the desulfurized fuel gas desulfurized in the desulfurizer 54 and then supplied thereto through a desulfurized gas supply line 59 provided with a desulfurized fuel gas on-off valve 55 into a hydrogen-rich reformed gas with reduced CO concentration by use of steam vaporized in a vaporizer 58 by supplying water thereto through a shut-off valve 57 and then supplied thereto through a check valve 59; and a fuel cell 63, which generates an electric power by electrochemically reacting the reformed gas obtained in the fuel reforming device 60 and then supplied to a fuel electrode (AN) through a reformed gas supply line 62 provided with a reformed gas on-off valve 61 with atmospheric oxygen supplied to an air electrode (CA).
The fuel cell power generation system 51 further comprises, as shown in FIG. 9, a combustion raw fuel gas supply line 66 branched from a branch part 64 of the raw fuel gas supply line 53 at the downstream of the raw fuel gas on-off valve 52 to supply part of the raw fuel gas to a combustion part (burner) 65 of the fuel reforming device 60 in order to supply a heat quantity necessary for maintaining the reforming reaction, because the reaction by steam reforming is an endothermic reaction. Hydrogen gas (offgas) discharged from the fuel cell 63 is supplied to the combustion part (burner) 65 through an offgas line 68 provided with a shut-off valve 67.
On the other hand, for avoiding the poisoning of the platinum catalyst by CO, it is considerable to use, as the catalyst for fuel electrode, a catalyst hardly poisoned by CO, for example, a platinum-ruthenium catalyst.
Ruthenium detoxifies CO. Therefore, the platinum-ruthenium catalyst is remarkably improved in the resistance to CO poisoning, compared with a single catalyst of platinum, and especially a platinum-ruthenium catalyst with ruthenium content of 50 weight % or more can exhibit a remarkable effect on the reduction in voltage by poisoning (refer to Japanese Patent Application Laid-Open No. 09 (1997)-35736).
However, ruthenium is low in hydrogen oxidation activity required as natural electrode reaction, compared with platinum. Although such platinum-ruthenium catalyst with ruthenium content of 50 weight % or more as described above is excellent in CO poisoning resistance, the resulting voltage is lower than that in a cell using the single catalyst of platinum when fuel composed of only hydrogen is used.
Therefore, a solid polymer type fuel cell comprising a fuel electrode having a first platinum-ruthenium catalyst layer with ruthenium content of less than 50 weight % on the side contacting with the polymer electrolyte membrane and a second platinum-ruthenium catalyst layer with ruthenium content of 50 weight % or more on the gas diffusion layer side is proposed (refer to Japanese Patent Application Laid-Open No. 10 (1998)-270057).
However, it was found that the platinum-ruthenium catalyst of the fuel electrode becomes single platinum by elution of ruthenium therein during operation using a fuel gas such as CO-containing hydrogen gas or an organic fuel such as methanol as the fuel, for example, during long-term operation involving frequently repeated starting and stoppage of the fuel cell. As a result, the CO resistance is deteriorated. Further, the dissolved and separated ruthenium can arrive at the air electrode through the electrolyte layer and inhibits oxygen reduction that is a reaction in the air electrode.
Therefore, such conventional electrode/membrane assembly with platinum-ruthenium catalyst and fuel cell power generation system using a fuel cell provided therewith could not perform stable generation of electric power due to the deterioration of CO poisoning resistance by elution of ruthenium and the inhibition of oxygen reduction that is the air electrode reaction by the eluted ruthenium, and thus lacked reliability.